1. Field of the Invention
This invention relates to a thermal displacement correcting method of correcting displacement, caused by heat, in a drive system of a tool machine.
2. Description of the Related Art
A driving system of a tool machine includes, for example, a motor serving as a driving source, and a driving shaft actuated by a driving force from the driving source, a table moved by the driving shaft. A workpiece is disposed on the table actuated by the driving system to be moved or positioned by driving the driving system.
In the driving system of the machine tool, heat may be generated from the driving source such as a motor, or heat may be generated due to friction or the like from a driving mechanism in a mechanical system such as ball screw interposed between the driving shaft and the table. The heat generation causes a variation in temperature in the driving system such as a driving shaft, and the variation in temperature results in thermal displacement in the driving system due to thermal expansion. The thermal displacement in the driving system generates deviation from a correct position when the table is positioned. Positioning error of the table affects a work positioning accuracy, and causes reduction of a work machining accuracy.
In the prior art, there are known methods of correcting the positioning error due to the thermal displacement, including, for example, the following methods:
1) A method having the steps of mounting a temperature sensor at each position of a mechanism portion in the driving system, calculating an amount of thermal displacement depending upon a temperature obtained from the temperature sensor, and correcting thermal displacement error in the driving system of the tool machine by using the amount of thermal displacement; PA1 2) A method having the steps of measuring a size of a finished work after machining by using a length measuring machine, indirectly finding an amount of thermal displacement of a machine tool depending upon the measured value, and correcting thermal displacement error in a driving system of the machine tool by using the amount of thermal displacement; and PA1 3) A method having the steps of measuring shaft positions of a machine tool before and after machining by using a position sensor or a length measuring machine, finding an amount of thermal displacement of the machine tool depending upon the measured values, and correcting thermal displacement error in a driving system of the machine tool by using the amount of thermal displacement.
FIG. 16 is an explanatory view of a method of correcting thermal displacement error in a driving system of a conventional machine tool, generally showing one embodiment of the driving system used for the machine tool. In FIG. 16, the driving system includes a motor 3 serving as the driving source, a driving shaft 1 connected to the motor 3 through a joint 4 and actuated by driving force of the motor 3, a nut 2 moved in conjunction with the driving shaft 1, a table 8 mounted on the nut 2 to support a work 9, and so forth. Further, the driving shaft 1 is mounted on a base 7 through a fixed bearing 5 and a supporting bearing 6.
For example, in the driving system used for the machine tool, temperature sensors 10 and 11 are mounted on the driving shaft 1, or the nut 2, thereby using the temperature sensors as means for finding the amount of thermal displacement. Thus, the thermal displacement error is found depending upon temperature measured by the temperature sensor. Alternatively, a position sensor 12 is mounted separately from the machine tool to find the thermal displacement error of the driving shaft 1 depending upon position data of the position sensor.
In each of the conventional methods of correcting the thermal displacement, there is inevitably generated a problem in that a detector such as temperature sensor or length measuring machine is required.